Known in the art is a process for producing sulphur from sulphates of alkali-earth metals of the Me.sub.X (SO.sub.4).sub.y type by way of treatment thereof with a gas mixture containing CO and CO.sub.2 at a temperature of 2,000.degree. to 3,000.degree. F. (1,090.degree.-1,650.degree. C.), a volume ratio of CO.sub.2 /CO of not less than 3 and at a volume concentration of CO of at least 5%; the molar ratio of (CO+CO.sub.2) to calcium sulphate being maintained equal to at least 3y:1, while the process being conducted under a pressure of from 0 to 25 psig (0 to 7 Atm) (cf. U.S. Pat. No. 3,726,957).
This prior art process has a disadvantage residing in that the reduction of sulphates of alkaliearth metals by carbon monoxide under the above-specified conditions at a temperature below the agglomeration temperature of the starting stock proceeds mainly according to the reaction (exemplified by calcium sulphate): EQU CaSO.sub.4 +CO=CaO+SO.sub.2 +CO.sub.2 (I),
sulphur is recovered predominantly as sulphur dioxide and its yield in elemental form is not high (40-50% of its content in the starting material) which makes the process economically ineffective.
Also known in the art is a process for producing sulphur from metal sulphates by treating particles of these raw materials in a furnace with two fluidized beds positioned one under another so that in the lower fluidized bed the raw materials are treated with a heated gas containing free oxygen, whereas in the upper fluidized bed consisting of a carbon-containing agent such as coke the reduction of sulphur dioxide formed in the lower bed is carried out to obtain elemental sulphur at a temperature of from 600.degree. to 1,250.degree. C. according to the reaction: EQU SO.sub.2 +C=S+CO.sub.2 (2)
(cf. U.S. Pat. No. 4,041,141).
A disadvantage of this process resides in that for the treatment of calcium sulphate it is necessary to keep high temperatures in the lower fluidized bed (within the range of from 1,250.degree. to 1,350.degree. C.) and use raw materials in the form of fine particles which results in a high degree of dusting of the reaction sulphur-containing gases, fusion and agglomeration of the raw materials containing contaminants which, in turn, inevitably impairs the fluidization conditions and results in stoppage of the formation of sulphur. Furthermore, the use of temperatures within the range of from 600.degree. to 1,250.degree. C. in the process to reducing of sulphur dioxide to elemental sulphur lowers the balanced yield of elemental sulphur.
Known in the art is a process for producing sulphur from calcium sulphate (a mixture of phosphogypsum and natural gypsum) involving granulation of a charge from powder-like calcium sulphate and a carbonaceous components such as coke or light-weight coals, heating of the resulting charge granules in a vertical calcination furnace at a continuous movement of these granules in the furnace in a continuous current directed downwardly with a simultaneous supply of a counter current of an oxygen-containing gas, such as air, so that maximum temperature of heating of the charge granules is maintained within the range of from 1,100.degree. to 1,200.degree. C., and condensation of sulphur vapours from the reaction sulphur-containing gases effluent from the furnace top section. In the stage of the charge granulation the mass ratio of carbon in the carbonaceous component to calcium sulphate is maintained equal to 0.25:1; the ash content of coke and light-weight coals is usually within the range of from 6 to 13% (cf. "Sulphur ", 1980, No. 147, p. 36-38).
This prior art process has a disadvantage residing in an insufficient yield of elemental sulphur (75% of its content in the starting material) due to the fact that the access of air oxygen into the granules during the heating is hindered and oxygen has not time to react with carbon according to the reaction: EQU 2C+O.sub.2 =2CO (3)
As a result, an excessive amount of calcium sulphide is formed in the granules which calcium sulphide prior to leaving the lower section of the furnace is enriched with oxygen of an oxygen-containing gas according to the reaction: EQU CaS+20.sub.2 =CaSO.sub.4 ( 4)
which lowers the yield of the sulphur product. Furthermore, impurities of phosphates and fluorides in phosphogypsum form liquid phases upon heating which migrate to the surface of the charge granules thus resulting in agglomeration of granules, hindering of reduction of calcium sulphate and reducing the yield of the final sulphur product.